In wireless communication systems, inevitable influence of terrains or obstacles on signals causes the occurrence of multipath distortion. As a time-varying channel impulse response is generally modeled as a time-domain discrete finite impulse response (FIR) filter denoted by
            h      ⁡              (                  τ          ;          t                )              =                  ∑        n            ⁢                                    α            n                    ⁡                      (            t            )                          ⁢                  ⅇ                                    -              j                        ⁢                                                  ⁢            2            ⁢            π            ⁢                                                  ⁢                          f              c                        ⁢                                          τ                n                            ⁡                              (                t                )                                                    ⁢                  δ          ⁡                      (                          τ              -                                                τ                  n                                ⁡                                  (                  t                  )                                                      )                                ,complex multipath interference always exists in received wideband signals, which appears as frequency selective fading in the frequency domain. For this reason, all receivers need to carry out an equalization operation to eliminate the multipath influence.
Generally, in a multipath channel, there will be a strongest path, called a main-path, which may be the earliest path or a path following several others. Paths earlier than the strongest path are defined as a forward path and those following the strongest path are defined as a backward path. The entire multipath transmission can be regarded as being composed of the forward path, the main-path and the backward path. These three components together determine the transmission performance of the system.
Shown in FIG. 1 is the frame structure of a 802.11b wireless LAN system. The frame mainly includes a preamble sequence, a frame header sequence and a data sequence. The preamble sequence is transmitted at 1 Mbps using differential binary phase shift keying (DBPSK) modulation and Barker code based spectrum spreading, followed by the frame header sequence, which may be transmitted at 1 Mbps using DBPSK modulation and Barker code based spectrum spreading, or at 2 Mbps using differential quadrature phase shift keying (DQPSK) modulation and Barker code based spectrum spreading. The portion of effective data may be transmitted either at 1 Mbps, 2 Mbps, 5.5 Mbps or 11 Mbps.
In 802.11b systems, channel estimation is often completed during the period of receiving the preamble. There are two kinds of preambles. The first one is referred to as short preamble and the second one is referred to as the long preamble. Duration of the long and short preamble is 144 μs and 72 μs, corresponding to 144 and 72 preamble bits, respectively. Further, preamble bit is composed of synchronous bit and check bit. Synchronous bit of a long preamble consists of 128 consecutive 1's, while that of a short preamble consists of 56 consecutive 0's. The length of check bit is 16 and check bits of a long and short preamble are different with each other. A receiver read a check bit to find whether the end of the current preamble is reached and whether the current preamble is a long or short one. Each preamble bit passes through a scrambling module such that any long string of consecutive 0' or 1' is eliminated and thus randomness of the preamble bit is strengthened. After that, the preamble bit is modulated according to the DBPSK modulation table (referring to Table 1) to generate a preamble symbol which has the same length with the preamble bit. Subsequently, each preamble symbol is spread with an 11-bit Barker code, bk=[+1 −1 +1 +1 −1 +1 +1 +1 −1 −1 −1], and is transmitted thereafter. In such a way, after modulation and Barker code based spectrum spreading, preamble sequences transmitted at a rate of 11 Mbps are obtained from preamble data transmitted at a rate of 1 Mbps.
TABLE 1DBPSK Modulation TableInput Bit01Phase Change0π
In a general case, channel estimation for an 802.11b system is achieved by cross-correlation of barker code, and the standard provides a Barker code auto-correlation function
      R    ⁡          (      k      )        =      {                                        11            ,                          k              =              0                                                                                      -              1                        ,                          k              ≠              0.                                          It could be found that the correlation function will give a peak wherever the start of a local Barker code is aligned with the start of a Barker code of a received sequence, otherwise less correction will be obtained.
Nevertheless, similar to other pseudo-random sequences, and also implied by the above auto-correlation function, a Barker code itself is not completely orthogonal, thus leading to impracticability of obtaining an accurate channel estimation response by directly carrying out the correlation with a local Barker code.